System.Security.Cryptography.CryptographicException: Key not valid for use in specified state

I used entlib 2.0 cryptography block to encrypt and decrypt user password, it runs well on my development but got a "System.Security.Cryptography.CryptographicException: Key not valid for use in specified state" error on my UAT.

Have no clue how this could happened and how to solve it. Any help will be appreciated.